Buffer amplifiers are used in a variety of applications to interface an input signal to a receiving circuit. For example, opamps are used in voltage-follower configurations with an inverting or ‘minus’ input terminal connected to an output so that the output voltage ideally tracks the input voltage signal. However, opamps loose gain as their output is driven towards a supply rail, such as ground, because the common-source output transistor goes into triode region operation when the associated drain-source voltage VDS is less than its saturation voltage VDSAT. Thus, despite connection in a voltage-follower configuration, the overall amplifier gain is not linear across the entire input signal range, and the gain is less than unity near the supply rails, for example 100 to 200 mV depending on bias and sizing of the output transistor. True unity gain can only be achieved in a middle range and the amplifier circuit suffers from gain error limiting the useful signal range.